1. Field of the Invention
This invention relates to a cassette for housing a stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, the phosphor emits light having intensity in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic material or on a display device such as a cathode ray tube (CRT).
This radiation image recording and reproducing system using the stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desired density regardless of the amount of exposure of the phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
The stimulable phosphor sheets used in the aforesaid radiation image recording and reproducing system are handled in the form housed in a cassette for containing a single stimulable phosphor sheet or in a magazine for containing many stimulable phosphor sheets. After radiation images are stored in the stimulable phosphor sheets, the cassettes or the magazine containing the stimulable phosphor sheets carrying the radiation images stored therein is loaded into a sheet feeding apparatus, i.e. a cassette feeder or a magazine feeder, from which the stimulable phosphor sheets are fed one by one to a radiation image read-out apparatus.
The aforesaid cassette used to house a single stimulable phosphor sheet is formed in the same manner as the conventional X-ray film cassette. That is, the cassette comprises a flat box-like front plate and a cover member for covering the front plate. The stimulable phosphor sheet is housed in the cassette so that the stimulable phosphor layer side of the stimulable phosphor sheet stands face to face with the inner surface of the front plate. Recording of a radiation image by use of a radiation such as X-rays is conducted by positioning the cassette housing the stimulable phosphor sheet as described above at a point exposed to the radiation emitted from a radiation source and passing through an object.
As described above, recording of a radiation image in the stimulable phosphor sheet is conducted by using the sheet in the form housed in the cassette. However, readout of the radiation image stored in the sheet is conducted by taking the sheet out of the cassette and scanning the sheet with stimulating rays.
In the radiation image read-out step, since the stimulable phosphor sheet is taken out of the cassette, the stimulable phosphor layer of the sheet housed in the conventional cassette is readily scratched by the inner surface of the front plate of the cassette when the sheet is taken out of the cassette. Further, the stimulable phosphor layer of the sheet is readily scratched by the inner surface of the front plate of the cassette also when the sheet is inserted into the cassette.
Also, in the conventional cassette, the stimulable phosphor sheet is closely contacted with the inner surface of the front plate of the cassette. Therefore, the removal of the sheet from the cassette by use of a vacuum attraction type suction arm or the like sometimes fails, and not infrequently the sheet is taken out of the cassette in an incorrect sheet position.